Add graphviz generation and visualization

This requires graphviz to be installed on your machine. If you run the
command with sudo, it will create the files with the original user
ownership to make it easier to remove them without root.

main.go

@@ -102,6 +102,12 @@ func run(c *cli.Context) {
 				log.Fatal("Graph failure")
 			}
 			log.Printf("Graph: %v\n", G) // show graph
+			err := G.ExecGraphviz(c.String("graphviz-filter"), c.String("graphviz"))
+			if err != nil {
+				log.Printf("Graphviz: %v", err)
+			} else {
+				log.Printf("Graphviz: Successfully generated graph!")
+			}
 			G.SetVertex()
 			if first {
 				// G.Start(...) needs to be synchronous or wait,
@@ -174,6 +180,16 @@ func main() {
 					Value: "",
 					Usage: "code definition to run",
 				},
+				cli.StringFlag{
+					Name:  "graphviz, g",
+					Value: "",
+					Usage: "output file for graphviz data",
+				},
+				cli.StringFlag{
+					Name:  "graphviz-filter, gf",
+					Value: "dot", // directed graph default
+					Usage: "graphviz filter to use",
+				},
 				// useful for testing multiple instances on same machine
 				cli.StringFlag{
 					Name:  "hostname",